Guardian Cosmos is the enigmatic guardian spirit of the Cosmos Crystal who resides within the Galaxy Cauldron. She only appears once in the very last chapter of the manga.

Guardian Cosmos is one of the most mysterious figures in the entire Sailor Moon manga. She appears to be the Sailor Power Guardian of the Cosmos Crystal and Sailor Cosmos and hence Usagi herself but only implies this. The manga hints this is made possible through a connection between herself, Queen Serenity, and the birth of Princess Serenity long ago but what exactly happened between them is left open to interpretation.

(Source: Sailor Moon Wiki)